1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Discussion of the Background
Duplex printing on electrographic image forming apparatuses involves printing (forming an image onto) one surface of a sheet of recording media at an image processor, reversing the sheet of recording media at a reversing unit, re-feeding the reversed sheet of recording media to the image processor, and printing the other surface of the sheet of recording media. When a plurality of sheets of recording media are duplex printed consecutively, a predetermined number of sheets among the plurality of sheets of recording media are grouped into one unit for one-surface printing and the other-surface printing. For example, assume that the predetermined number is four. In this case, four sheets of recording media are first subjected to one-surface printing consecutively, next circulated and reversed, and then subjected to the other-surface printing consecutively.
As used herein, the predetermined number of sheets refers to the maximum number of sheets of recording media simultaneously containable in the path through which the recording media are circulated (the maximum number being hereinafter referred to as the circulated sheet number). The circulated sheet number is basically determined based on the entire length of the path in relation to the length of a sheet of recording media in the conveyance direction.
For example, Japanese Unexamined Patent Application Publication No. 2002-37540 discloses an image forming apparatus that sets a circulated sheet number based on the size and type of recording media, and consecutively duplex prints a plurality of sheets of the recording media corresponding to the circulated sheet number that has been set.
Generally, the entire length of the path is set to direct highest duplex productivity to the circulation of most frequently used recording media (for example, A4 in portrait). Thus, the path is designed based on the circulation of A4 in portrait. For example, when the circulated sheet number of A4 in portrait is five, the entire length of the path is set to ensure that printing one surface of the fifth sheet of recording media is immediately followed by printing the other surface of the first sheet. The duplex productivity is high when during consecutive duplex printing, the interval of conveyance between the last and first sheets of recording media is not significantly different from the corresponding interval in consecutive simplex printing. That is, high duplex productivity is when consecutive duplex printing shows a similar level of efficiency to the level of efficiency shown by consecutive simplex printing (simplex productivity). When A4 in portrait is conveyed, its shorter sides are orthogonal to the conveyance direction and the longer sides are 210 mm in the conveyance direction.